1. Field of the Invention
The present invention relates to a process and an apparatus for uniformly heat-treating a substrate which has a film-forming composition thereon.
2. Discussion of the Related Art
There are known substrates having films or layers formed thereon of a given material such as a metallic or inorganic material. In the present specification, the term xe2x80x9cfilmxe2x80x9d and the term xe2x80x9clayerxe2x80x9d are used interchangeably, unless otherwise specified. Such substrates include glass substrates made of a glass material, typically, a soda-lime glass, and ceramic substrates made of a ceramic material, typically, alumina. A film or layer, which has a certain function, may be bonded to the substrate by fusion or melting of a glass bonding component or by softening, melting or sintering of the material per se. These substrates may be used for anode plates for vacuum fluorescent displays (VFD), plasma switching boards for plasma address liquid crystal displays (PALC), field-emission displays (FED) and other display devices, thick-film wiring boards, and various electronic devices such as thermal printer heads and image sensors. Generally, the substrates for these electronic devices are subjected to heat treatments at temperatures of about 500-650xc2x0 C. for the purpose of annealing the substrates per se or forming functional films with a glass material used as a bonding agent. Where the substrates are ceramic, the substrates are heat-treated at about 500-900xc2x0 C. for forming functional films with a glass material used as a bonding agent or for forming functional films of a metallic material by utilizing the fusion of the metallic material at the interface with the substrates.
Recently, there have been increasing requirements for increasing the number of conductive, resistive, dielectric and other layers or films formed in desired patterns, and for increasing the density of such layers or films. Further, there has been an increasing demand for display devices having a large-sized display screen, and an accordingly increasing requirement for increasing the size of the substrates for such large-sized display devices. To meet these requirements, it is required to form minutely patterned layers or films over a comparatively large area, particularly, on the substrates for the display devices. The substrates for electronic devices described above have patterned functional films having minute cells or cavities. To assure high dimensional and positional accuracy of these minute cells, the functional films should be patterned with an improved degree of uniformity. However, the above heat treatment or firing of the substrates has an influence on the quality of the substrates, which influence increases as the size of the substrates increases. Therefore, the heat treatment causes a variation in the quality of the products using the substrates, and provides some restrictions in the design of the products, or reduces the yield of the products. The quality variation may be a variation in the resistance value of a resistor film, a variation in the withstand voltage of a dielectric film, a variation in the thickness due to uneven ratio of removal of binders by firing of the dielectric film, a variation in the continuity or resistance of a conductive film, a variation in the ease of wire-bonding or sputtering on the conductive film.
Where the substrate suffers from a dimensional change due to expansion or shrinkage of its material upon heat treatment, it is difficult to accurately position the patterned functional films relative to each other, since each function film is fired after it is formed in a predetermined pattern. The uniformity and positioning accuracy of the patterned films tend to be deteriorated with an increase in the density (minuteness) and size of the substrate, whereby the yield ratio of the product is significantly lowered as the density or size of the substrate is increased. In the case of a substrate for a plasma display device having a screen size as large as 40 inches, for example, the causes for lowering the yield ratio may include: insufficient dimensional accuracy of multiple layers which form multiple cells; variation in the height and width dimensions of partition walls; variation in the resistance of resistor cells; variation in the withstand voltage of a dielectric layer; an overall dimensional variation; and inaccurate positioning of front and rear plates which form a discharge cell.
The present invention was made in the light of the prior art drawbacks described above. It is therefore a first object of this invention to provide a process of firing a substrate with a film-forming composition provided thereon, which permits uniform heating of the substrate to thereby assure a high yield ratio of a product including the substrate.
It is a second object of the invention to provide a firing apparatus suitable for practicing the method indicated above.
The first object may be achieved according to a first aspect of the present invention, which provides a firing process of uniformly heat-treating a substrate having a film-forming composition thereon, comprising the steps of: (a) a first soaking step of holding, for a predetermined first time, said substrate in a first heating chamber whose temperature is maintained at a predetermined first value, so that the temperature within said substrate is held at said first value evenly throughout an entire mass of said substrate; (b) a feeding step of feeding said substrate subjected to said first soaking step, to a second heating chamber whose temperature is maintained at a predetermined second value which is different from said predetermined first value by a predetermined difference; and (c) a second soaking step of holding said substrate in said second heating chamber for a predetermined second time, so that the temperature within said substrate is held at said second value evenly throughout the entire mass of said substrate.
In the present firing process, the substrate having a film-forming composition is heat-treated by first subjecting the substrate to the first soaking step in the first heating chamber in which the substrate is held at the predetermined first temperature value for the predetermined first time for even distribution of the temperature throughout the entire mass of the substrate. Then, the substrate is fed into the second heating chamber whose temperature is maintained at the predetermined second value different from the first value by the predetermined amount, and is subjected to the second soaking step in which the substrate is held at the second value for the predetermined second time for even distribution of the temperature throughout the entire mass of the substrate. The substrate may be subjected to a further soaking step or steps. Thus, the substrate is heat-treated at different temperatures which are different from each other, so that a local variation in the temperature within the substrate and the film-forming composition is minimized. Where the substrate is formed of a glass material and is heat-treated at temperatures higher than the strain point of the glass material, a local dimensional variation or configurational deviation of the substrate can be minimized. Accordingly, the present firing process permits accurate positioning accuracy of films, layers or any other structural features subsequently formed on the substrate, resulting in a considerably increased yield ratio of the product which includes the substrate, even where the substrate has minute or intricate structural patterns or has a relatively large size. The film-forming composition provided on the surface of the substrate may be thick-film dielectric films, dielectric partition walls, thick-film resistor films, electrode films or inorganic pigment films. Since the present firing process makes it possible to minimize the local temperature variation within the substrate and within such films or layers formed thereon, a glass material contained in the films or layers as a bonding component may be uniformly melted or softened in the heat treatment process, and a metallic material or metal oxide contained in the films may be uniformly melted or sintered. Accordingly, the product has reduced variation in its properties such as withstand voltage, resistance value, discharge characteristics and optical filter characteristics and in its dimensions such as height and width dimensions of the partition walls. Consequently, the yield ratio of the product is significantly improved, even where the substrate has a large size. Further, the reduced variation in the resistance value results in a reduced cost of control of the production steps and elimination of some steps such as trimming step.
In the present process, the first and second temperature values are preferably determined to be close to the transition or strain point of a glass material contained in the substrate so that the temperature of the substrate changes through the transition or strain point while the temperature within the substrate is evenly distributed throughout the entire mass of the substrate. Where the films are bonded to the substrate by melting or sintering of a metallic or inorganic material, the first and second temperature values are preferably determined to be close to the melting or sintering point of the metallic or inorganic material so that the temperature of the substrate changes through the melting or sintering point while the temperature within the substrate is evenly distributed throughout the entire mass of the substrate.
In one preferred form of the first aspect of the invention, the firing process further comprises a first stand-by step which is implemented concurrently with the first soaking step, to adjust the temperature in the second heating chamber to the predetermined second value so that the second soaking step is implemented in the second heating chamber, and further comprising a second stand-by step which is implemented concurrently with the second soaking step, to adjust the temperature in the first heating chamber to a predetermined third value which is different from the predetermined second value by a predetermined difference, so that a third soaking step is implemented in the first heating chamber. In this case, the temperature in the second heating chamber is maintained at the second value while the first soaking step is implemented, and the temperature in the first heating chamber is maintained at the third value while the second soaking step is implemented. Accordingly, the second and third soaking steps may be initiated immediately after the termination of the first and second soaking steps, respectively.
In one advantageous arrangement of the above first preferred form of the firing process, the first stand-by step comprises a cooling step of lowering the temperature in the second heating chamber to a value lower than the predetermined second value by a predetermined amount, and a temperature raising and holding step of effecting feed-back control to raise the temperature in the second heating chamber to the predetermined second value and maintain the second value, and wherein the second stand-by step comprises a cooling step of lowering the temperature in the first heating chamber to a value lower than the predetermined third value by a predetermined amount, and a temperature raising and holding step of effecting feed-back control to raise the temperature in the first heating chamber to the predetermined third value and maintain the third value. In this arrangement, the temperature in the second heating chamber is first lowered below the second value and then raised to the second value while the first soaking step is implemented in the first heating chamber. Similarly, the temperature in the first heating chamber is first lowered below the third value and then raised to the third value while the second soaking step is implemented in the second heating chamber. This arrangement permits the second and third temperature values to be rapidly established in the first and second stand-by steps.
The second object indicated above may be achieved according to a second aspect of this invention, which provides a firing apparatus for uniformly heat-treating a substrate having a film-forming composition thereon, comprising: (a) shutter devices which partially define a first and a second heating chamber such that said first and second heating chambers are thermally insulated from each other; (b) a heating device for controlling temperatures in said first and second heating chambers, independently of each other; (c) a feeding device for feeding said substrate into said first and second heating chambers alternately, so that said substrate is heat-treated in said first and second chambers alternately; and (d) a control device for controlling said heating device to maintain the temperature in said first heating chamber at a predetermined first value while said substrate is heat-treated in said first heating chamber, and adjusting the temperature in said second heating chamber to a predetermined second value different from said predetermined first value by a predetermined difference while said substrate is heat-treated in said first heating chamber, said control device adjusting the temperature in said first heating chamber to a predetermined third value different from said predetermined second value by a predetermined difference while said substrate is heat-treated in said second heating chamber, so that said substrate is then heat-treated in said first heating chamber at said third value.
In the present firing apparatus, the substrate having a film-forming composition is heat-treated in the first heating chamber at the predetermined first temperature value for the predetermined first time for even distribution of the temperature throughout the entire mass of the substrate. Then, the substrate is fed by the feeding device into the second heating chamber the temperature of which is maintained at the predetermined second value different from the first value, and is heat-treated in the second heating chamber at the second value for the predetermined second time for even distribution of the temperature throughout the entire mass of the substrate. The temperature in the first heating chamber is adjusted to the predetermined third value different from the second value while the substrate is heat-treated in the second heating chamber, and the substrate is then heat-treated in the first heating chamber at the third value. Thus, the substrate is alternately placed in the first and second heating chambers, and heat-treated there at different temperatures, so that a local variation in the temperature within the substrate and the film-forming composition is minimized. Where the substrate is formed of a glass material and is heat-treated at temperatures higher than the strain point of the glass material, a local dimensional variation or configurational deviation of the substrate can be minimized. Accordingly, the present firing apparatus permits accurate positioning accuracy of films, layers or any other structural features subsequently formed on the substrate, resulting in a considerably increased yield ratio of the product which includes the substrate, even where the substrate has minute or intricate structural patterns or has a relatively large size. The film-forming composition provided on the surface of the substrate may be thick-film dielectric films, dielectric partition walls, thick-film resistor films, electrode films or inorganic pigment films. Since the present firing apparatus makes it possible to minimize the local temperature variation within the substrate and within such films or layers formed thereon, a glass material contained in the films or layers as a bonding component may be uniformly melted or softened in the heat treatment process, and a metallic material or metal oxide contained in the films may be uniformly melted or sintered. Accordingly, the product has reduced variation in its properties such as withstand voltage, resistance value, discharge characteristics and optical filter characteristics and in its dimensions such as height and width dimensions of the partition walls. Consequently, the yield ratio of the product is significantly improved, even where the substrate has a large size. Further, the reduced variation in the resistance value results in a reduced cost of control of the production steps and elimination of some steps such as trimming step.
Further, the present firing apparatus uses only the two heating chambers for heat treatment of the substrate, namely, only the first and second heating chambers to which the substrate is alternately fed for heat treatment. Accordingly, the longitudinal dimension of the firing apparatus is advantageously reduced.
In the present firing apparatus, the first, second and third temperature values are preferably determined to be close to the transition or strain point of a glass material or the melting or sintering point of a metallic or inorganic material so that the temperature of the substrate changes through the transition, strain, melting or sintering point indicated above, while the temperature of the substrate is evenly distributed throughout the substrate, as described above with respect to the firing process.
In one preferred form of the firing apparatus according to the second aspect of this invention, the apparatus further comprises a cooling device for lowering the temperatures in said first and second heating chambers. In this case, the temperature in the stand-by heating chamber in which the substrate is not currently heat-treated may be adjusted to the predetermined value, by first operating the cooling device to positively lower the temperature in the stand-by heating chamber to a level lower than the predetermined value and then controlling the heating device to adjust the temperature in the stand-by heating chamber to the predetermined value. The initial operation of the cooling device and the subsequent operation of the heating device permit rapid and uniform adjustment of the temperature in the stand-by heating chamber to the predetermined value at which the substrate is then heat-treated.
Preferably, the cooling device comprises cooling tubes for delivering cooling air into the first and second heating chambers, so that the temperature in the stand-by heating chamber is lowered by the cooling air delivered by the cooling tubes.
In another preferred form of the above firing apparatus, each of the shutter devices includes a shutter member movable between an open position and a closed position for thermal insulation of the first and second heating chambers. The shutter member is placed in the open position when the substrate is fed by the feeding device into or from the first or second heating chamber, and in the closed position while the substrate is heat-treated in one of the first and second heating chambers and while the temperature in the other heating chamber is adjusted. Since the shutter members of the shutter devices assure thermal insulation of the first and second heating chambers, the temperature can be evenly or uniformly distributed within the heating chambers during heat treatment of the substrate or during adjustment of the temperature in the stand-by heating chamber. Accordingly, the present arrangement permits a further reduced local variation in the temperature within the substrate.
The second object indicated above may also be achieved according to a third aspect of this invention, which provides a firing apparatus for uniformly heat-treating a substrate having a film-forming composition thereon, comprising: (a) shutter devices which partially define at least two heating chambers including a first and a second heating chamber such that said at least two heating chambers are thermally insulated from each other; (b) a heating device for controlling temperatures in said first and second heating chambers independently of each other; (c) a temperature control device for controlling said heating device to maintain the temperature in said first heating chamber at a predetermined first value uniformly throughout said first heating chamber, and to maintain the temperature in said second heating chamber at a predetermined second value uniformly throughout said second heating chamber, said second value being different from said first value by a predetermined difference; and (d) a feeding device for feeding said substrate in one feeding direction, first into said first heating chamber for heat-treating said substrate at said predetermined first value for a predetermined first time, and then into said second heating chamber for heat-treating said substrate at said predetermined second value for a predetermined second time, said feeding device further feeding said substrate from said second heating chamber after heat treatment thereof in said second heating chamber.
In the present firing apparatus according to the third aspect of the invention, the substrate having a film-forming composition is heat-treated in the first heating chamber at the predetermined first temperature value for the predetermined first time for even distribution of the temperature throughout the entire mass of the substrate. Then, the substrate is fed by the feeding device into the second heating chamber the temperature of which is maintained at the predetermined second value different from the first value, and is heat-treated in the second heating chamber at the second value for the predetermined second time for even distribution of the temperature throughout the entire mass of the substrate. Thus, the substrate is heat-treated in the first and second heating chambers at different temperatures so that a local variation in the temperature within the substrate and the film-forming composition is minimized. Where the substrate is formed of a glass material and is heat-treated at temperatures higher than the strain point of the glass material, a local dimensional variation or configurational deviation of the substrate can be minimized. Accordingly, the present firing apparatus permits accurate positioning accuracy of films, layers or any other structural features subsequently formed on the substrate, resulting in a considerably increased yield ratio of the product which includes the substrate, even where the substrate has minute or intricate structural patterns or has a relatively large size. The film-forming composition provided on the surface of the substrate may be thick-film dielectric films, dielectric partition walls, thick-film resistor films, electrode films or inorganic pigment films. Since the present firing apparatus makes it possible to minimize the local temperature variation within the substrate and within such films or layers formed thereon, a glass material contained in the films or layers as a bonding component may be uniformly melted or softened in the heat treatment process, and a metallic material or metal oxide contained in the films may be uniformly melted or sintered. Accordingly, the product has reduced variation in its properties such as withstand voltage, resistance value, discharge characteristics and optical filter characteristics and in its dimensions such as height and width dimensions of the partition walls. Consequently, the yield ratio of the product is significantly improved, even where the substrate has a large size. Further, the reduced variation in the resistance value results in a reduced cost of control of the production steps and elimination of some steps such as trimming step.
Further, the substrate is heat-treated at the different first and second temperature values in the first and second heating chambers while it is fed in one direction by the feeding device. Accordingly, the overall length of the present firing apparatus can be made smaller than that of a conventional continuous feeding type firing apparatus which is adapted to continuously feed the substrate so as to cool the temperature of the substrate according to a continuous temperature cooling pattern that permits the substrate to have an extremely reduced local temperature variation. Since the present apparatus does not have a stand-by heating chamber as provided in a shutter type apparatus in which the substrate is reciprocated between two heating chambers, the present apparatus provides an accordingly increased degree of heat treating efficiency and is suitable for mass production of a product using the substrate.
In one preferred form of the present apparatus according to the third aspect of the invention, each of the shutter devices includes a shutter member movable between an open position and a closed position for thermal insulation of the first and second heating chambers. The movable shutter member is placed in the open position when the substrate is fed by the feeding device into or from the first or second heating chamber, and in the closed position while the substrate is heat-treated in the first or second heating chamber. Since the shutter members of the shutter devices assure thermal insulation of the first and second heating chambers, the temperature can be evenly or uniformly distributed within the heating chambers during heat treatment of the substrate. Accordingly, the present arrangement permits a further reduced local variation in the temperature within the substrate.
In another preferred form of the present apparatus, the feeding device comprises a plurality of rollers whose axes of rotation are parallel to each other and perpendicular to the above-indicated one feeding direction and which are arranged in this feeding direction to support the substrate. The rollers are rotated to feed the substrate in the feeding direction. In this arrangement, the substrate is supported by the plurality of rollers and fed in the predetermined feeding direction with the rollers being rotated. Thus, the rollers are used in place of a generally used endless belt made of a mesh of refractory metal, for example. In the present arrangement, the two or more heating chambers including the first and second heating chambers and the feeding device provide a roller hearth kiln for firing the substrate having films formed thereon. In this roller hearth kiln, the films formed on the substrate are less likely to be adversely influenced by dust which may be considerably scattered in a heating area using a conveyor belt. Namely, the feeding of the substrate by the rotating rollers is less likely to deteriorate the function of the films on the substrate due to dust during the heat treatment therein.
In one advantageous arrangement of the above preferred form of the apparatus, one of the shutter devices includes a shutter which is movable in a vertical direction perpendicular to the feeding direction, between an open position and a closed position, through a gap between adjacent ones of the plurality of rollers. The shutter placed in the closed position separates the first and second heating chambers from each other with thermal insulation therebetween. In this arrangement, the shutter is vertically movable without an interference with the rollers, permitting complete thermal insulation of the first and second heating chambers, and assuring improved uniformity of temperature in each heating chamber and accordingly reduced local variation of the temperature within the substrate.
In another advantageous arrangement of the above preferred form of the apparatus, each of the plurality of rollers is made of a ceramic material. In this case, the rollers are less likely to be worn, rusted, damaged or deteriorated due to contact with the substrate and heating in the heating chambers, assuring an reduced amount of dust produced in the heating chambers and accordingly enhanced quality of the fired substrate.
Where the rollers are made of a ceramic material, each of the above-indicated two heating chambers has an inner wall surface preferably made of a ceramic material, and the shutter of each shutter device is also preferably made of a ceramic material. Thus, the rollers of the feeding device, the shutters and the inner wall surfaces of the heating chambers are all made of the ceramic material, and are less likely to be worn, rusted, damaged or deteriorated due to heating, assuring a further reduced amount of dust produced in the heating chambers.
In a further preferred form of the firing apparatus according to the third aspect of the invention, the feeding device includes an intermittently feeding device for intermittently feeding the substrate by rotation of the plurality of rollers through the at least two heating chambers, and a continuously feeding device for continuously feeding the substrate by rotation of the rollers at a predetermined feeding speed through a continuous heat treatment zone which includes an area adjacent to the at least two heating chambers. The continuously feeding device includes a feeding speed changing device for changing rotating speeds of the rollers in the above-indicated area so that a feeding speed of the substrate in this area is almost equal to the feeding speed by the intermittently feeding device.
In the above preferred form of the firing apparatus, the substrate is fed intermittently through the at least two heating chambers, and is continuously fed at a given speed through the continuous heat treatment zone. In the above-indicated area of the continuous heat treatment area adjacent to the first heating chamber, for example, the rotating speed of the rollers and the feeding speed of the substrate are raised to those of the intermittently feeding device, so that the substrate may be smoothly and relatively rapidly fed from the above area into the first heating chamber, so that the time required for the substrate to move between the above area and the first heating chamber is shortened, making it possible to reduce the local variation of the temperature within the substrate due to a difference in the temperatures between the above area of the continuous heat treatment zone and the first heating chamber, for example. Further, the feeding speed changing device is effective to reduce an amount of sliding movement between the rollers and the substrate due to the difference in the feeding speed between the above area and the first heating chamber, whereby the amount of dust produced in the furnace is accordingly reduced. It is also noted that the shutters of the shutter devices are placed in their open position for a time as short as possible, so as to minimize a deviation of the temperature in each heating chamber from the predetermined values and an uneven temperature distribution within each heating chamber.